1. Field of the Invention
The present invention relates to an optical fiber amplifying apparatus, and more particularly to an optical fiber amplifying apparatus capable of operating stably against variations in device temperature or in signal light level.
2. Description of the Related Prior Art
In a wavelength-multiplex optical transmission system, it is possible to expand the transmission capacity of a single optical fiber transmission path by multiplexing signal lights of a plurality of wavelengths. When the number of signal lights is increased, it becomes necessary for the optical fiber amplifier applied to this system to be flat in the wavelength-dependence of the gain in a wide range of wavelengths. Generally, optical fiber amplifiers applied to a wavelength-multiplex optical transmission system uses a 1550-nm band, commonly known as the C band, and a 1580-nm band, known as the L band, in both of which the wavelength characteristic of the gain is flat in a wide range of wavelengths. However, an optical fiber amplifier using an erbium-doped optical fiber (EDF) as the amplifying optical fiber involves the problem that the gain/wavelength characteristic of the EDF in the L band is heavily dependent on temperature. As a consequence, temperature fluctuations in the environment in which such an optical fiber amplifier is installed invite variations in the gain/wavelength characteristic of the optical fiber amplifier. For instance, even if the output level of each of the wavelength multiplex signal lights after amplification is substantially the same and flat, any fluctuation in temperature around the device may give rise to wavelength-dependence in the gain of the optical fiber amplifier, resulting in differences in output level among the individual signal lights after they are amplified.
Therefore, an object of the present invention is to provide an optical fiber amplifier for use in wavelength-multiplex optical transmission systems, more particularly an optical fiber amplifier which is susceptible to no variation in the wavelength-dependence of the gain or output level against fluctuations in ambient temperature, signal light level or the like and is capable of stable operation.
An optical fiber amplifying apparatus according to the invention is provided with an optical fiber (EDF) doped with some rare earth for amplifying input signal lights and a temperature control means for controlling the temperature of the optical fiber, and directly amplifies the input signal lights. The temperature control means can be provided with a temperature sensor for detecting the temperature around the optical fiber, a heating/cooling element for heating or cooling the optical fiber, and a temperature control circuit for controlling the heating/cooling element according to the output of the temperature sensor. Instead of the heating/cooling element, an electroconductive film formed over the surface of the optical fiber and a current applying means for flowing an electric current to the electroconductive film can be used.
Another optical fiber amplifier according to the invention is provided with a signal light extracting means for extracting at least one signal light out of the amplified wavelength-multiplex signal lights and a level detecting means for detecting the level of the extracted signal light. In this optical fiber amplifier, the temperature control means is provided with a heating/cooling element for heating or cooling the optical fiber and a temperature control circuit for controlling the heating/cooling element according to the level of the detected signal light. The temperature control circuit controls the heating/cooling element so as to keep the level of the signal light at a prescribed value.
An optical fiber amplifier according to still another aspect of the invention is provided with, in addition to the signal light extracting means, a signal light extracting means, arranged on the input side of the optical fiber, for extracting at least one signal light out of the amplified wavelength-multiplex signal lights and a level detecting means for detecting the level of the signal light extracted from the signal light extracting means. The two signal light extracting means extract signal lights of the same wavelength, and the temperature control circuit controls the heating/cooling element on the basis of the two levels of the extracted signal lights. More specifically, the temperature control circuit computes the gains of the signal lights from the two levels of the signal lights, and controls the heating/cooling element so as to keep the gains at a prescribed value. Where a plurality of signal lights are extracted, the temperature control circuit controls the heating/cooling element so as to equalize the gains of the signal lights.
The optical fiber (EDF) for amplifying the signal lights are arranged to be in close contact with the optical fiber (EDF), a bobbin or a member having a grooved part. The heating/cooling element, which may be a Peltier element for instance, is arranged in contact with the bobbin or the member. The signal light extracting means may be an optical branching unit, consisting of a combination of an optical branching device and an optical filter, a variable-wavelength optical filter, a fiber grating or an arrayed waveguide grating.
Such an optical fiber amplifying apparatus can operate stably against variations in ambient temperature or in input signal light level.